Device for measuring the angle of movement of two vertebrae

ABSTRACT

A device for measuring angles of freedom of adjacent vertebrae which includes two reference elements associated with each vertebra respectively. Indicators are associated with the two reference elements for measuring the angle of freedom of movement of the vertebrae in torsion in a plane perpendicular to the longitudinal axis of the vertebrae and for measuring the angle of freedom of the vertebrae in lateral inflexion in a plane perpendicular to that of the spinous processes as a function of a given position of flexion-extension of the vertebrae in the plane containing the spinous processes.

This application is a division, of application Ser. No. 07/950,632,filed Sep. 14, 1992, now U.S. Pat. No. 5,291,901.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a device intended to measure, on theone hand, the angle of freedom of two adjacent vertebrae in torsion and,on the other hand, the angle of freedom of these vertebrae in lateralinflexion with respect to a given position in flexion-extension of thevertebrae.

HISTORY OF THE RELATED ART

In theory, it is considered that the intervertebral articulationcomprises 6° of freedom, but, in practice, the principal freedom of thearticulation lies in flexion-extension. Similarly, but to a lesseramplitude, there exist degrees of freedom upon intervertebral horizontalrotation or lateral flexion.

It should also be noted that these two movements are usually linked witheach other, i.e. the lateral flexion necessarily causes a certainhorizontal degree of rotation, and vice versa.

The problem of this lumbar intervertebral articulation is that it israpidly subject to damage, leading to a reduction in the rubbery qualityof the disc. Such deterioration results in a greater suppleness of thearticulation, which brings about more ample movements both inflexion-extension, but also in lateral inflexion and in torsion.

The reduction in the rubbery quality of the disc leads to a completedisorganization of the movements of the lumbar intervertebralarticulation.

It therefore seems important, for certain therapeutical decisions, bothto be able to measure the amplitude of all the intervertebral movementsand to have an idea of the disorganization of the coupling of themovements with respect to one another.

It is a particular object of the present invention to provide a solutionto this problem.

SUMMARY OF THE INVENTION

The purpose of the present invention is to design a measuring devicecapable of indicating, at any moment and in any position, thethree-dimensional rotations of one vertebra with respect to the other.This device makes it possible in particular to assess the movements ofintervertebral inflexion and rotation at certain places of theintervertebral flexion-extension.

The measurements obtained by this device will make it possible to takedecisions for certain therapeutical positions, and this "in vivo",during operations. The measuring device may be used outside surgery,abutting percutaneously on the intervertebral spines. During anoperation, the measuring device will abut on screws which will be placedin the pedicles of the vertebra.

The device according to the invention for measuring the amplitudes oftwo vertebrae comprises two reference elements rigidly associatedrespectively with each vertebra, means associated with two referenceelements for measuring the angle of freedom of the vertebra consideredin torsion, i.e. in a plane perpendicular to the longitudinal axis ofthese vertebrae, and other means associated with such elements formeasuring the angle of freedom of the vertebrae in lateral inflexion,i.e. in a plane perpendicular to that of the spinous processes as afunction of a given position in flexion-extension of the vertebrae, i.e.in the plane containing the spinous processes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood on reading the followingdescription with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of the device according to the invention.

FIG. 2 is a plan view of this device.

FIG. 3 is a view similar to that of FIG. 1, but Illustrating twoadjacent vertebrae displaced angularly with respect to each other in thesense of a cyphosis.

FIG. 4 shows the measurement of the angle of twist of the two vertebraefrom their position illustrated in FIG. 3.

FIG. 5 is a view similar to that of FIG. 2, but illustrating themeasurement of the angle of displacement of the two vertebrae in lateralinflexion from their position of FIG. 3.

FIG. 6 is a perspective view of a variant embodiment of the device ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 illustrates two adjacent vertebrae1,2 separated in conventional manner by a disc 3. X-Y designates thelongitudinal axis of the vertebrae and a plane P containing this axis isshown, as well as the spinous processes 1a, 2a of the vertebrae 1 and 2.This Figure also shows a transverse plane Q passing through the disc 3andorthogonal with respect to the axis X, Y as well as to plane P. Athird plane R containing axis X, Y has also been shown in FIG. 1, itsorientation being orthogonal with respect to the two planes P and Q.

In the following specification, the term flexion-extension willdesignate the movement of the vertebrae in position, for example, ofcyphosis in which the axis X-Y becomes curved while remaining in planeP. The term torsion will designate the rotation of one vertebra withrespect to the other planes parallel to plane Q. Finally, lateralinflexion will designate the movement of the vertebrae with respect toeach other such that axis X-Y becomes curved while remaining in plane R.

The device according to the invention firstly comprises two small rods4, 5of which one of the ends is respectively fixed to each of thevertebrae 1 and 2 by any appropriate means, for example implants orscrews 6, 7. The two small rods rise slightly obliquely in order thatthe free end of column 4 is offset laterally with respect to plane Pwhose trace has been shown in FIG. 2, while the free end of column 5 isvirtually contained in this plane.

The free end of rod 4 comprises an endpiece 4a of smaller diameter,which engages freely in a hole of a block 8 having a small bar 9extending parallel to column 4. This bar includes two pivots 9a, 9b.

The upper part of rod 5 is secured to an oblique flat section 10 towhich is secured a rod 11 extending substantially parallel to axis P(cf. FIG. 2). On this rod is mounted to slide freely a carriage 12 towhich is secured a segment of circle forming dial 13 which is suitablygraduated and parallel to plane P. On its lateral face, the carriage 12includes a pivot 12a parallel to those 9a, 9b of bar 9. A suppleconnecting rod 14 isarticulated on pivots 9b, 12a. About the latterpivots the apex of a squarebracket 15 of which one, 15a, of the arms,parallel to bar 9 and of the same length, is provided with a pivot pin16 for one of the ends of a supple connecting rod 17 of which the otherend is articulated on pivot 9a. The other arm 15b of the square bracket15 constitutes a needle which moves in front of the graduated portion ofthe dial 13. The two connectingrods 14, 17, the bar 9 and the arm 15 aof the needle 15 form a deformable parallelogram linkage.

The upper part of the column 4 located immediately below the block 8 isprovided with an oblique flat section 18 secured to a rod 19 extendingperpendicularly to rod 4 and virtually parallel to rod 11, asillustrated in FIG. 2. A carriage 20 is mounted to slide freely alongrod 19. This carriage bears a suitably graduated dial 21 disposedparallel to plane R as well as a pin 20a about which is articulated aneedle 22 of which the end opposite that located in front of thegraduation of dial 21 is articulated about the free end 5a of rod 5.

A bearing 23, substantially parallel to plane R, is fixed to section 18,its orientation being perpendicular to the plane formed by rod 4 and rod19. In this bearing is mounted for free rotation the end of one of thearms 24a of a square-bent rod 24 of which the second arm, parallel torod 19, receives for free sliding movement a block 25 bearing agraduated dial26 oriented substantially parallel to plane R. This blockis also secured to one of the ends of a bar 27 of which the other endincludes a head 27a traversed for free sliding movement by a rod 28secured to the second rod 5. Rod 28 presents the form of a squarebracket of which the arm 28a whichcooperates with the head 27a isperpendicular to the plane determined by the rod 5 and the rod 11. Thearm 28a of the rod 28 is freely slidable with respect to a block 29. Thelatter, the block 25 and the head 27a respectively carry pivots 29a, 25aand 27b. Between pivots 27b and 29a is mounted a lever 30. A lever 31 isarticulated by one of its ends on pivot 29a and by its other end on apin 32 a needle 33 of which the end oppositeits indicator part isarticulated about pivot 25a. In this way, the bar 27,the levers 30 and31 and the needle 33 form a deformable parallelogram.

Operation is as follows: during the measurement of the amplitude of theclearance of the two vertebrae 1 and 2 in the sense offlexion-extension, the curvature of the vertebrae in this sense createsan angle α (FIG. 3) between rods 4 and 5, so that the end 5a of rod 5moves away fromthat, 4a, of rod 4 (arrow F₁), with the columns beingdisplaced in planes parallel to plane P. Such displacement provokes amovement of the carriage 12 along the rod 11, so that the deformableparallelogram formed by bar 9, the two supple rods 14, 17 and the arm15a of the needle 15 is deformed, bringing about the displacement of theend of the needle 15 withrespect to the graduation of the dial 13 toindicate the value of the angle α. Of course, this displacement alsocauses the carriage 20 to slidewith respect to rod 19, but withoutnoteworthy modification of the positionof the needle 22. Simultaneously,the rod 28 tips in the direction of the vertebrae, which provokes arotation of the rod 24 in the bearing 23. Due to the parallelism betweenrods 11 and 19, the position of the needle 33 with respect to thegraduation of the dial 26 does not visibly vary.

From any relative position of flexion-extension of the two vertebrae 1and 2, it is possible to measure the amplitudes in rotation and inlateral inflexion of the two vertebrae.

If the vertebrae are displaced in lateral inflexion, i.e. giving axisX-Y acurvature in plane R, the two rods 4 and 5 effect complexrotations, so that, on the one hand, carriage 20 slides along rod 19 dueto the spaced apart relationship of the two rods 4 and 5 and, on theother hand, rod 28 changes relative position with respect to rod 24,moving away from it in the direction of arrow F2 (FIG. 5), while tippingin the direction of arrow F3. Under these conditions, the deformableparallelogram 27, 30, 31,33 is deformed and the end of the needle 33moves with respect to the graduation of the dial 26 to indicate themaximum angle of clearance of the two vertebrae in lateral inflexion.

The maximum angle of rotation of one vertebra with respect to the otherin plane Q may also be measured. The relative displacement of the tworods 4 and 5 in this plane Q, in the case of lateral flexion, causes thetwo ends4a and 5a of the columns to move apart, which causes, on the onehand, movement of the carriage 20 on rod 19 and a subsequent rotation ofthe needle 22 whose end moves in front of the graduation of the dial 21to indicate, for example, the maximum amplitude of freedom of the twovertebrae in the direction of the rotation.

Under these conditions, the utility of providing the supple connectingrods14 and 17 so that they may follow without deformation the relativemovements of the rods 4 and 5 in rotation and in lateral inflexion, willbe understood.

Of course, proximity gauges or potentiometers may be provided at thepins 12a of carriage 12, 20a of carriage 20 and 25a of block 25 tomeasure the angles of clearance by means of an electronic converter withdigital display or like apparatus to which the gauges or potentiometerswould be connected.

FIG. 6 illustrates a variant embodiment of the measuring deviceaccording to the invention.

The device illustrated in FIGS. 1 to 6 allows the measurement of theanglesof torsion, of lateral inflexion and of flexion-extension ofadjacent vertebrae but without reference to the force applied on thesevertebrae todisplace them in the three directions mentioned above.

On the contrary, in accordance with the device of FIG. 6, it is possibletomeasure the angles of torsion and of flexion-extension of thevertebrae 1 and 2 as a function of the force exerted thereon via therods 4 and 5 which each include a strain gauge 34. This gauge is placedin the vicinityof attachment of rods 4 and 5 with respect to the screwsor implants 6, 7 connected to the vertebrae 1 and 2. Rods 4 and 5 are infact constituted by rods bent twice as an S and which are disposedsubstantially in the plane P containing the longitudinal axis X-Y of thespine.

The ends 4a, 5a of the rods which are located substantially parallel toscrews 6 and 7 bear at their ends a rotation monitoring gauge 35 adaptedto rotate in a block 36. The opposite faces of the two blocks 36 eachinclude a fork joint 36a in which is articulated one of the ends of aconnecting rod 37, 38 respectively. The opposite end of the connectingrod38 is secured to a rotation monitoring gauge 39 rotating with respectto a pin fixed to the corresponding end of the other connecting rod 37.In thisway, rods 37 and 38 substantially form compasses of which thelegs are displaced in plane P.

Gauges 34, 35 and 39 are connected to an electronic converter 40 withdigital display adapted to display the angles of torsion and offlexion-extension of the vertebrae 1 and 2 as a function of the forceapplied thereon by displacing rods 4 and 5 either in torsion or inflexion-extension.

This measurement is therefore effected for example in torsion bydisplacingthe two rods 4 and 5 in two opposite directionsperpendicularly to plane P.This displacement provokes rotation of therotation monitoring gauges 35 with respect to the blocks 36, the blocksremaining in plane P due to their link with connecting rods 37 and 38.

If it is desired to measure the angle of flexion-extension, rods 4, 5are displaced in plane P in opposite direction, which provokesdisplacement ofthe two blocks 36, moving away from each other so that,in that case, it isgauge 39 which measures the variation of the angle ofthe connecting rods.

It goes without saying that the proximity gauges may be replaced byrotatable potentiometers without departing from the scope of theinventionor by any other like device.

What is claimed is:
 1. An apparatus for measuring an angle of movementof one vertebra of a patient with respect to another spaced vertebrarelative to at least two of three orthogonal planes which extend througha longitudinal axis along which the vertebrae are aligned for purposesof making therapeutical decisions with respect to spinal surgicalprocedures, wherein the vertebrae include outwardly extending spinousprocesses and wherein the three orthogonal planes include a first planeextending along the longitudinal axis and in general alignment with thespinous processes, a second plane extending perpendicularly with respectto the longitudinal axis, and a third plane orthogonal to both the firstand second planes and extending along the longitudinal axis, theapparatus comprising: an electrical converter and display means forindicating a degree of flexion-extensional movement in said first planeof the spaced vertebrae with respect to one another and indicating adegree of torsional movement of the spaced vertebrae with respect to oneanother in planes parallel to said second plane, first and second rodseach having lower ends for engaging respective vertebrae in spacerelationship with respect to one another and each having upper ends,each of said rods including a strain gauge spaced intermediate saidupper and lower ends, means for electrically connecting said straingauge to said electrical converter and display means, a pair of firstrotation monitoring gauges mounted in spaced relationship to each otheron said upper ends of said rods a pair of spaced block members, a pairof additional rods each having first ends mounted in spaced relationshipto one of said block members and each supporting said block members insurrounding relationship to one of said first rotation monitoringgauges, each of said additional rods having second ends which arepivotable about a joint with respect to one another, a second rotationmonitoring gauge mounted to measure the relative rotation between saidsecond ends of said additional rods, and means for electricallyconnecting each of said first and said second rotation monitoring gaugesto said electrical convertor and display means whereby the degree offlexion-extensional movement of the spaced vertebrae relative to oneanother and the degree of torsional movement of the spaced vertebraewith respect to one another may be displayed when forces are applied onsaid first and second rods.